Optical demultiplexer modules have a function to demultiplex waveform-multiplexed light, and are capable of receiving a multiplexed light signal and producing output signals separated in respective channels. Generally, input light is introduced from a single optical fiber into an optical demultiplexer module, which properly separates the input light into signals of respective channel wavelengths therein, and applies the signals to a plurality of respective optical fibers or photodetector devices for respective separate channels, thereby producing output signals in the respective separate channels.
The wavelength band mainly used in the field of optical communications at present is a 1550 nm band with channels separated at frequency intervals of 100 GHz. If the frequency intervals are expressed in terms of wavelength pitches, then the signals in the channels are arranged at wavelength intervals of about 0.8 nm. An optical demultiplexer module is required to angularly separate the signals with a diffraction grating and apply them accurately to different optical fibers or photodetector devices in respective channels. Since the wavelength intervals are small, the light beams emitted in the respective channels from the diffraction grating have small angular differences.
For assembling an optical demultiplexer module, an active alignment process is carried out by fixing the components thereof in respective given positions such that light having waveforms corresponding to respective channels is introduced from an input fiber and outputted accurately from output fibers or photodetector devices. The alignment process needs time, and a very large expenditure of time and labor is required to perform the alignment process. Highly accurate and expensive apparatus are needed to assemble optical demultiplexer modules.
Also, an optical demultiplexer module is made up of many components and is assembled according to a complex assembling procedure. Since it is particularly necessary to take care of variations in characteristics and dimensions of the components and slight environmental changes, an assembling algorithm is not easy to determine, it is difficult to automatize the assembling process, as a result of which skilled workers need to work on the components, resulting in difficulties with the mass production of optical demultiplexer modules.
If a passive alignment process is employed in lieu of the active alignment process, then errors with respect to dimensions of various components and devices and assembling errors, represented by the following accuracies, are accumulated:                (1) the cutting accuracy of the diffraction grating with respect to the direction of grooves;        (2) the mounting accuracy of a photodetector array chip;        (3) the accuracy of the package of a photodetector array;        (4) the dimensional accuracy of a casing; and        (5) the assembling and bonding accuracy of the above components.        
It is thus difficult to achieve a desired performance level with the passive alignment process which depends on abutment of the components, for aligning and assembling optical demultiplexer modules which require a high level of accuracy.